Cell-Cycle Controls in G1 and G0

Abstract

The process of cellular division is divided into four phases, termed G1 phase, S phase, G2 phase and M phase. In the G1 phase, two critical processes are performed. First, the cell undergoes extensive increase in mass. Second, it makes the decision to cross the growth factor-dependent restriction point, a critical checkpoint. Tumor cells select for genetic mutations that disrupt the important decisions performed at the restriction point and, this is, in fact, one of the hallmarks of cancer. One of the key negative regulators of the restriction point is the retinoblastoma tumor suppressor protein (Rb). Rb is regulated by a group of serine/threonine-specific protein kinases called cyclin-dependent kinases (Cdks). The balance between Rb and Cdks determined by the extent of extracellular growth factor stimulation of receptors ultimately decides if the cell will divide or not.